1. Field of the Invention
The present invention relates to a method of forming a luster coating film on a substrate.
2. Description of Related Art
Luster coating films usually contain flaky luster pigments, such as aluminum flakes, mica flakes, etc., and have various color tones. Such luster coating films shine brilliantly as the luster pigments reflect incident light from outside the coating films, and exhibit a unique and variable aesthetic appearance achieved by the combination of the reflected light and the color tones of the coating films.
In recent years, luster coating films, in particular those formed on automotive bodies or the like, are being required to exhibit high-quality appearance characteristics, such as a highly dense texture, high flip-flop property, etc. As used herein, dense texture means uniform, continuous luster created by a luster pigment in a coating film. A highly dense texture can be achieved when a coating film has little graininess caused by the luster pigment contained therein. Flip-flop property is produced by the orientation of the flaky luster pigment in a coating film, parallel to the coating film surface. A coating film with that property reflects light well and has high brightness in the highlight (i.e., when viewed from the front), but has low brightness in the shade (i.e., when viewed at an angle). That is, flip-flop property is a property that produces a difference in brightness depending on the angle of vision.
Japanese Unexamined Patent Publication No. 2002-273333 discloses a method of forming a luster coating film, in which a luster base coating is formed on a substrate in two stages, followed by forming a clear coating, the thickness ratio of the base coating composition applied in the first stage to that applied in the second stage being 2/1 to 4/1. This method can prevent unevenness in the luster of luster coating films.
However, especially when an aqueous luster base coating composition is used, the above method is likely to result in insufficient orientation of the flaky luster pigment used, and is not capable of forming a luster coating film with a highly dense texture and high flip-flop property.